JAPAN’S effort to bring its nuclear crisis and feared meltdown under control could take weeks, a senior US nuclear expert has warned.

The warning came as the Group of Seven countries agreed to a joint intervention to counter a sharp rise in the yen following last week’s massive earthquake and ensuing tsunami.

The operators of Japan’s quake-damaged Fukushima nuclear plant today continued their bid to douse fuel rods and prevent a disastrous release of radiation, following some early signs of success. The Japanese military yesterday used trucks and helicopters to dump tonnes of water onto overheating reactors at the nuclear plant in a desperate effort to contain the nuclear crisis.

Early today plant operator Tokyo Electric Power Co said the operation was believed to have had some effect. “When we poured water, we monitored steam rising from the facility. By pouring water, we believe the water turned down the heat. We believe that there was a certain effect,” a spokesman said.

He said TEPCO and the military team planned to keep pouring water on to the stricken plant today.  The operation by plant workers aims to keep the fuel rods inside reactors and containment pools submerged under water, to stop them from degrading when they are exposed to air and emitting dangerous radioactive material.

However Greg Jaczko, chairman of the US Nuclear Regulatory Commission, said the operation to draw heat from the damaged reactors could take some time, and the crisis could be prolonged.  “This is something that will likely take some time to work through, possibly weeks, as eventually you remove the majority of the heat from the reactors and then the spent fuel pool,” he said.

The UN nuclear agency also warned earlier that the situation at Fukushima remained “very serious”. Chief government spokesman Yukio Edano said it would take some time to find out whether the cooling operation had led to any significant improvement. “Based on what experts have told us, it’s important to have a certain level of water (in the pools) before we can start to see any positive effect,” Mr Edano told reporters.

“We are closely monitoring the data,” he said.  As Japanese authorities continue to try to control the release of radiation at Fukushima, 250km northeast of Tokyo, more foreign nationals are fleeing as governments advise their citizens to leave. Airline tickets have sold out and private jet firms have been swamped with bookings.

Australians are among those told to leave Tokyo due to the radiation risk from the earthquake-damaged nuclear reactors. The Department of Foreign Affairs and Trade has upgraded its travel advice, which also sets an 80-kilometre exclusion zone around the power plant – much wider than the 20km exclusion zone set by Japan.

The first flight chartered by the United States took off from Japan for Taiwan yesterday, carrying mostly families of US personnel and some other Americans. Another flight was due to leave Japan today.

With Japan’s economy also under siege, the G7’s announcement that the Japanese, US, eurozone, Canadian and British authorities would intervene later today in the foreign exchange market immediately pushed down the yen and helped to lift battered Tokyo shares.

The developments came as President Barack Obama ordered a “comprehensive review” of US nuclear safety and vowed to learn lessons from Japan’s nuclear accident. Mr Obama also sought to reassure nervous Americans, saying he did not expect harmful radiation from Japan to reach the US or any of its territories.

Paving the way for a more direct role by the US military, the Pentagon said it had sent a team of experts to evaluate what assistance US forces could provide to the Fukushima plant.

Japanese Defence Minister Toshimi Kitazawa said more military vehicles would be deployed to help with efforts to cool the reactors at Fukushima, while pumps supplied by the US armed forces were also being transferred.

The Japanese government’s nuclear safety agency has said the top priority should be pouring water into the fuel-rod pools at reactors three and four, which may be boiling and are not fully covered by roofs that would reduce radiation leaks.  A TEPCO official said the pool at theNo 4 reactor “seemed to have water” on Wednesday, based on aerial observations carried out by the military helicopters.

Another TEPCO spokesman said: “We have not confirmed how much water was left inside but we have not had information that spent fuel rods are exposed.” Workers are also continuing with the crucial task of trying to restore power lines, in a bid to reactivate crippled cooling systems knocked out by last Friday’s earthquake and resultant tsunami.

The nuclear safety agency said early today that TEPCO had managed to get a line from a regional power firm into the plant site which would allow it to restore the cooling system.  “But the line has yet to reach the reactors’ power system and it will take 10 or 15 hours to connect the line to it,” an agency spokesman said.

A TEPCO spokesman said earlier:”If the restoration work is completed, we will be able to activate various electric pumps and pour water into reactors and pools for spent nuclear fuel.” The 9.0-magnitude quake, the biggest on record to strike Japan, knocked down electricity pylons which were used to supply power to the TEPCO plant.

Some 70 workers have been using pumps to pour seawater to cool reactors at the plant, according to media reports, using electricity from borrowed mobile generators.  In Fukushima prefecture, where a zone within 20 kilometres of the plant has already been evacuated, about 10,000 people were to be screened at 26 locations for radiation exposure, Kyodo News reported, citing local officials.

So far, radioactivity has been detected on six people, whose faces and hands were wiped clean, the report said.

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